Adding to the growing compendium of Kuiper belt objects, astronomers have spotted two new moons orbiting Pluto. Observations with the Hubble Space Telescope from May of last year show two tiny dots revolving around the same center of gravity as the ninth planet and its largest moon, Charon. Reporting the finding today in Nature, the researchers speculate that the tiny companions formed in the same cataclysmic collision that produced Charon.
"We used Hubble's exceptional resolution to peer close to Pluto and pick out two small moons that had eluded detection for more than 75 years," says Hal Weaver, an astrophysicist at Johns Hopkins University and the discovery team leader. "That was somewhat surprising because ground-based observers had been trying for more than a decade to find new satellites around Pluto," adds astronomer Max Mutchler of the Space Telescope Science Institute, the first to see the moons in Hubble's images.
Based on their brightness--and assuming that their surfaces are about as reflective as Charon's--the scientists believe the two moons are roughly 38 miles and 29 miles in diameter. Given that they share Pluto's distance from the sun--roughly three billion miles--but are 4,000 times fainter, it is not surprising that the satellites eluded detection until now, the researchers say.
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Dubbed S2005 P1 and S2005 P2 while awaiting proper names, the two moons share the same orbital plane as Charon, albeit with much longer orbits. The team thus suspects that they may have formed from the same collision--imagined by an artist in the image above--that is theorized to have created the binary Pluto-Charon planetary system. The moons may also be a source for dust that coalesces into rings around Pluto before dispersing. "If Pluto's small moons generate debris rings from impacts on their surfaces, as we predict, it would open up a whole new class of study because it would constitute the first ring system seen around a solid body," notes Bill Merline, team member from the Southwest Research Institute.
The New Horizons mission--launched on January 19 and scheduled to fly by Pluto in July 2015--will provide the definitive glimpse of these newest members of the solar system. But in the interim, additional observations by Hubble may provide more details. Ultimately, it may prove that Kuiper belt objects like Pluto, 2003 UB313, and the Centaurs--which have been pushed into orbits closer to the sun--typically have multiple moons. Or even rings.
